Structural relaxation and nanocrystallization of FeCoBSiNbCu bulk metallic glass: A comprehensive time-resolved synchrotron X-ray diffraction study

Metallic glasses and their resulting nanocrystalline composites reveal very favorable properties that have been recognized from the early stage of their development. Of vital importance is the structure at the nanoscale and the possibility of tailoring it. Controlled nanocrystallization is one prerequisite for metallic glass functionalization, but this method cannot be extensively applied without a priori knowledge of the phase formation. Moreover, in the case of well-established characterization methods only a very small amount of material is generally analyzed, where it is questionable whether the bulk material properties are addressed. In this study we apply in situ synchrotron X-ray diffraction to achieve accurate details on the structural relaxation and evolution of an (Fe,Co)-based bulk Finemet-type alloy. Via comprehensive analysis of both the reciprocal- and direct-space data, we show that the Fe₃₅.₈Co₃₅.₈B₁₉.₁Si₄.₈Nb₄.₀Cu₀.₅ BMG rods with 2 mm diameter reveal just minimal structural relaxation prior to the glass transition (T_g). In turn, immediately above T_g, α-Fe-like nanocrystals start to form and undergo a continuous change in their chemical composition. Upon temperature increase, the first atomic shell does not expand as long as the alloy is still in the amorphous state, but shrinks as nanocrystallization proceeds.